Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus. The disease, which was first identified in Italy more than 100 years ago, occurs worldwide. Sixteen subtypes of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses potentially circulating in bird populations. To date, all known outbreaks of the highly pathogenic form have been caused by influenza A viruses of subtypes H5 and H7.
Of the 16 avian influenza virus subtypes, H5N1 is of particular concern for several reasons. H5N1 mutates rapidly and has a documented propensity to acquire genes from viruses, thereby facilitating infection of other animal species. Indeed, its ability to cause severe disease in humans has now been documented. Laboratory studies have demonstrated that isolates from this virus have a high pathogenicity and can cause mortality in humans.
Two other avian influenza viruses have recently been found to cause illness in humans: H7N7 and H9N2.
All type A influenza viruses are genetically labile and well adapted to elude host defenses. Influenza viruses lack mechanisms for the “proofreading” and repair of errors that occur during replication. As a result of these uncorrected errors, the genetic composition of the viruses changes as they replicate in humans and animals, and new antigenic variants emerge. These constant, permanent and usually small changes in the antigenic composition of influenza A viruses are known as antigenic “drift”.
Influenza viruses are typed as A or B on the basis of relatively stable intracellular nucleoproteins and envelope associated matrix proteins. Virus subtypes are based on two proteins in the viral envelope, hemagglutinin (HA) and neuraminidase (NA), which undergo constant antigenic change. 16 distinct subtypes of HA and 9 subtypes of NA are recognized for influenza A viruses. The sudden appearance of a new subtype (antigenic shift) has caused three major pandemics in the past century: 1918 (Spanish Flu, H1N1), 1957 (Asian Flu, H2N2) and 1968 (Hong Kong Flu, H3N2).
Influenza viruses have a second characteristic of great public health concern: influenza A viruses can swap or “re-assort” genetic materials between subtypes of any species resulting in novel subtypes. This reassortment process, known as antigenic “shift,” has resulted in worldwide pandemics in humans.
Influenza pandemics have occurred, on average, three to four times each century when new virus subtypes have emerged that are readily transmitted from person to person. In the 20th century, the great influenza pandemic of 1918-1919, which caused an estimated 40 to 50 million deaths worldwide, was followed by pandemics in 1957-1958 and 1968-1969. Experts surmise that another influenza pandemic is inevitable and possibly imminent. Given the unpredictable behaviour of influenza viruses, neither the timing nor the severity of the next pandemic can be predicted with any certainty.
Seven variable B-cell epitopes, and one variable T-cell epitope collectively represent the antigenic drift sites found on the hemagglutinin HA1 protein of Influenza A (subtype H5). Each of the B-cell variable epitopes represents a conformational epitope, and four of them are comprised of two discontinuous stretches of amino acids. There are two extended antigenic sites on the HA1 proteins, and each of them is represented by two distinct peptide sequences. The nonadjacent segments (stretches of amino acids) that are artificially joined together to represent the discontinuous epitopes are selected using the three-dimensional structure of A/duck/Singapore/3/97 hemagglutinin (PDB ID code: 1JSM). Use of crystallographic data aids in design of linear peptides that can mimic the native conformational epitopes of proteins. The T-cell eptiope is represented by a linear peptide sequence which may also be lipidated.
To date, no effective peptide-based vaccine against avian influenza is commercially available.
Current antiviral therapies may be clinically effective against influenza A virus strains in otherwise healthy adults and children; however, these therapies have limitations. Some of these drugs are expensive and supplies are limited. The vaccine composition must also change each year to account for changes in the virus circulating in the population due to antigenic drift. At least four months of development time is required to produce a new effective vaccine in significant quantities.
Processes for preparation of an immunogenic peptide mixture are described by Torres in U.S. Pat. No. 7,118,874, and in PCT application PCT/CA06/000891, herein incorporated by reference. According to one of these processes, the variability of immunogenic epitope sequences of a pathogen are evaluated. A peptide mixture is synthesized comprising a plurality of peptides representative of the frequency with which different amino acids are found at variable residues of selected epitopes.
Thus, there is a need to develop a vaccine formulation effective against multiple subtypes and multiple variants of avian influenza.